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Weeds are trying to corner the market on your soil. Left unchecked, a single waterhemp plant can flood your weed seed bank with hundreds of thousands of seeds. Without timely postemergence control and layered herbicide protection, you could be financing next year’s weed takeover.
Postemergence weed control is a balancing act to protect the yield of this season as well as future productivity. To be effective, post applications must be timely and layered with the right residuals, said Kurt Maertens, technical agronomist for BASF in western Illinois and eastern Iowa.
“We can’t give weeds a break, not even once, especially waterhemp,” he said. “Once the canopy closes and shades the soil surface, we stop new weeds from emerging. That’s the name of the game: Lay down enough residual herbicide and clean up any escapes, so we end up with clean fields.”
The Ultimate Goal
Effective weed control has two overarching goals, said Erin Burns, assistant professor of integrated weed management at Michigan State University. “The first is immediate: to protect crop yield this year,” she said. “Your long-term goal, especially with our more troublesome weeds, is to deplete the weed seed bank.”
Letting even a few waterhemp or Palmer amaranth escape can dramatically increase future weed pressure, Burns explained. “Just one waterhemp plant can produce up to a million seeds per plant, many of which remain viable in the soil for up to four years,” she said. “The weed seed bank can quickly explode, causing a problem long term.”
Timing Is Critical
In a 34-site-year study across the Midwest, researchers found that controlling weeds when they were just 2 inches tall preserved 100% of yield potential. But by 4 inches, already 3% of yield was lost. Waiting until 6 inches in height cost yield 6%, and by 9–12 inches, losses accelerated dramatically, Burns said.
Herbicide labels list the maximum weed height they can control. For example, glyphosate is labeled to control 20-inch lambsquarters. Yield loss occurs well before weeds reach that size, however. Because weed growth can be rapid, scouting frequently and targeting weeds at 2–4 inches is critical for protecting yield, Burns said.
Maertens agreed. “With waterhemp, you can go from 2 inches to 6 inches in just a couple of days when conditions are warm and wet,” he said. “If you wait, you’ll not only lose yield, but you’ll need higher herbicide rates for control.”
Layer Residuals to Outlast Weeds
One of the most important postemergence strategies is to use layered residuals. Waterhemp has an extended emergence pattern that can outlast a preemergence residual, Burns explained.
More growers are realizing they need to overlap their herbicide programs, Maertens said: “That means coming back with a post pass that not only cleans up what’s emerged but also adds a second layer of residual to protect against future flushes.”
Choosing the right post herbicide depends heavily on your weed spectrum and the crop’s trait package. Waterhemp in Midwest states commonly resists ALS inhibitors and glyphosate, with some populations also resistant to atrazine and PPO inhibitors.
To manage resistance, Burns recommends using at least two effective sites of action in postemergence corn programs. Group 4 herbicides, such as 2,4-D or dicamba, are effective, as are HPPD inhibitors (Group 27), including Armezon, Impact, and Laudis. Including atrazine (Group 5) with HPPDs can further enhance control. Diversifying herbicide modes of action is key to effective and sustainable waterhemp management.
Greeneye Technology
In soybeans, options depend on the traits, Burns said. PPO inhibitors (Group 14) work across varieties. Liberty (glufosinate) can be used on LibertyLink or XtendFlex varieties. Enlist soybeans allow the use of 2,4-D choline (Enlist One or Duo) and Liberty, offering two effective sites of action, Burns said. This diversification is important, especially since Group 14 resistance is present in Michigan. Without a herbicide-tolerant trait package — non-GMO soybeans — postemergence options are limited.
Group 15 herbicides may be added to the post pass to provide residual control, but they do not affect emerged weeds. Those residuals are especially helpful for late-emerging waterhemp after rain. However, Burns reminded farmers to watch for maximum use rates and rotation restrictions; some Group 14 herbicides can limit planting wheat for up to four months.
Weather Wild Card
The weather plays a huge role in post success. Delaying application due to rain can allow weeds to grow past ideal height and reduce herbicide efficacy. “If weeds grow beyond label size, you must adjust the rate,” Maertens said.
For example, if waterhemp escapes grow 2-6 inches while you wait for a spray window, bumping up Liberty Ultra from 24 to 29 fluid ounces per acre may be needed, Maertens said. In corn, Status may need to go from 5 to 7 ounces per acre, depending on weed size.
Rainfall intervals also matter. “Some herbicides, like glyphosate and 2,4-D, need about four hours of dry time to be effective,” Burns said. “Others, like Gramoxone, need as little as two. Knowing that matters in a rainy forecast.”
Field Edges Matter
If post programs are stopping weeds in the row but ignoring edges, the seed bank persists, Maertens said. “Weeds that go to seed on the margins of fields can be pulled through the combine and distributed across the whole field,” he explained.
Mechanical control, such as mowing, hand-roguing, or even targeted spot-spraying 2,4-D after brown silk in corn, can prevent seed from creeping in from the margins. “I prefer 2,4-D amine plus crop oil for edge cleanup,” Maertens said, “because it’s less volatile than ester formulations.”
Investing Now Saves Later
While it’s tempting to cut weed control costs in a tight year, Maertens warned that skipping a step could cost more in the long run.
“Weed control is always a good investment,” Maertens said. “If you let it go this year, you’re guaranteeing you’ll spend more next year — and potentially reduce yield, too.”
“The goal,” Burns said, “is to take as much seed out of the soil seed bank as possible — and put nothing new in.”
